The fabrication of integrated circuit devices typically requires a series of masks by which images can be projected to pattern a substrate. Due to the increasing complexity of integrated circuit designs, as well as the shrinking geometries of the resulting circuit, it is often desirable to perform a pilot production for verification before full-scale production is initiated. It is desired to facilitate pilot production in a manner that reduces costs and materials, and provides a relatively quick time to full-volume production.
In full-volume production, a single mask (in the present disclosure, the terms mask, photomask, and reticle are used to refer to the same item) is used to produce multiple instances of an image, such as for multiple instances of the same layer on multiple die. In this way, multiple die can be exposed at the same time. One common procedure used in pilot production is to use multi-layer masks (MLMs). In an MLM, instead of multiple instances of the same image, a single mask will have different images corresponding to different layers. With MLM, a first layer is exposed by one image, the MLM is moved relative to the substrate, and a second layer is then exposed. For the sake of example, and not intended to be limiting, a mask can include four images for exposing four die images of a common layer at once. A multi-layer mask (MLM) will have four images, one for each of four different layers on the chip. While throughput is reduced using a MLM (in the above example, only one die image is produced at a time), the reduction in cost of producing one reticle instead of four can be very beneficial.
MLMs are shown in prior art U.S. Pat. Nos. 6,710,851 and 5,995,200, both of which are hereby incorporated by reference. Both of these patents describe multiple images on a single reticle, with the images separated by perimeter borders and interstitial borders. However, these MLMs cannot be used in full-volume production. It is desired to improve on the MLMs shown in these prior art patents.